Missing rbd API components: RBD_LOCK_MODE_EXCLUSIVE_TRANSIENT

```
 typedef enum {
   RBD_LOCK_MODE_EXCLUSIVE = 0,
   RBD_LOCK_MODE_SHARED = 1,
+  RBD_LOCK_MODE_EXCLUSIVE_TRANSIENT = 2
 } rbd_lock_mode_t;
```
This just landed in main and is on its way to squid (planned in 19.2.4) and tentacle (planned in 20.2.1).